Is there any festival in your country which directly related with brother and sister. In India their are two festivals:
i) Bhai phota (in somepart called Bhau beej or bhai dooj) celebrated in the last day of Diwali festival. On this day, sisters pray for their brothers to have long and happy lives by performing the Teeka ceremony, and brothers make gifts to their sisters.
ii) Raksha bandhan celebrated in the Shravan Purnima. The central ceremony involves the tying of a rakhi (sacred thread) by a sister on her brother's wrist. This symbolizes the sister's love and prayers for her brother's well-being, and the brother's lifelong vow to protect her.